For more than 30 years, electrical stimulation of nervous tissue has been used to control chronic pain. Therapy originates from an implanted source device, called an electric signal generator. The electrical signals, usually a series of brief duration electrical pulses, are delivered through one or more implanted leads that communicate with the source device, and contain several conductive metal electrodes to act as low impedance pathways for current to pass to tissues of interest. For example, in spinal cord stimulation (SCS) techniques, electrical stimulation is provided to precise parts of the human spinal cord through a lead that is usually deployed in the epidural space dorsal to the spinal cord. Such techniques have proven effective in treating or managing disease and chronic pain conditions.
Percutaneous leads are small diameter leads that may be inserted into the human body through a Tuohy (non-coring) needle, which includes a central lumen through which the lead is guided. Percutaneous leads are advantageous because they may be inserted into the body with a minimum of trauma to surrounding tissue. On the other hand, the designs of lead structure that may be incorporated into percutaneous leads are limited because the lead diameter or cross-section must be small enough to permit the lead to pass through the Tuohy needle, generally less than 2.0 mm diameter. Typically, the electrodes, also called contacts, on percutaneous leads are cylindrical metal structures, with a diameter of approximately 1.0 mm and a length of 4.0 to 10.0 mm. Of course, half of each of these electrodes, facing away from the tissue of interest, is not very useful in delivering therapeutic current. Thus the surface area of electrodes that face the tissue to be excited is small, typically 3.0 to 10.0 square mm. Electrodes must be approximately this size for many human applications, especially SCS, to allow sufficient charge to be delivered with each electrical pulse to excite cells, but without a high charge density (charge/pulse/square mm) that might damage tissue or the electrode itself.
Paddle leads, like Model 3596 Resume® Lead, Model 3982 SyrnMix® Lead or Model 3991 Transverse Tripole® Lead of Medtronic, Inc., have been developed to offer improved therapy control over some aspects of percutaneous leads. Paddle leads include a generally two-dimensional array of electrodes on one side of an insulative body, for providing electrical stimulation to excitable tissue of the body. A paddle design allows electrodes to be considerably wider than percutaneous leads, up to 4.0 mm or more. Two-dimensional arrays of electrodes allow programming of active sites and better control of the electric field distribution.
One disadvantage recognized in known paddle leads is that their installation, repositioning and removal necessitates laminectomy, which is a major back surgery done by neurosurgeons and orthopedic surgeons, involving removal of part of the vertebral bone. Laminectomy is required because paddle leads have a relatively large width (up to 1.0 cm or more) compared to percutaneous leads. Thus, implantation, repositioning or removal of a paddle lead requires a rather large opening between the vertebral bones.
Electrodes on paddle leads can easily have larger surface areas than percutaneous leads, typically 8.0 to 20.0 square mm or more. Such electrodes are mainly circular or rectangular, and require welds to fine, flexible wires passing through the length of the lead body. Such welds are prone to breakage in high flex situations unless a relatively thick paddle is used to shield the welds and support the electrodes. One advantage of preferred embodiments of the invention is that welds are not required in places where they might encounter flexing.
Because of the size and relative stiffness of paddle leads compared to the tissues they lie near to, more scar tissue or fibrosis tends to form around them over time than around percutaneous leads. This can reduce electrical efficiency, and lead to the need for larger currents over time. Such scar tissue also necessitates greater surgical efforts for removal of paddle leads, if required. On some occasions, physicians have even clipped off the lead body and left a paddle permanently in a patient rather that surgically remove it, if the system should cease giving therapeutic benefit.
For these above listed benefits and liabilities, there is a need for a lead that can be percutaneously inserted through a Tuohy-type needle, but which can create electrodes, each with substantial 2-dimensional surface area, at positions that are more lateral than the current percutaneous lead bodies. Furthermore, if such a lead could be safely removed by simple traction on the lead body, the increased surgical efforts that are required of paddle-type leads could be avoided.
The prior art has shown some examples of leads that can be expanded in situ, but they cannot perform all of the above listed features.
Mullett in U.S. Pat. No. 5,121,754 described a percutaneously-inserted epidural stimulation lead that can be straightened by a stylet and inserted into the epidural space through a Tuohy needle, and then will assume a sigmoidal shape that had been preset in it once the stylet is removed. This allows a plurality of electrodes to be positioned at a variety of longitudinal and lateral positions over the dorsal surface of the spinal cord. Because each electrode is a cylindrical metal electrode of fixed size and shape, the device cannot reliably place several electrodes at each longitudinal position. With a diameter less than 2.0 mm, each electrode must have a length of several millimeters to pass adequate currents for SCS (typically 10-20 milliamperes). Hence on such simple percutaneous leads the electrodes are manufactured as metal cylinders whose diameter matches the lead diameter. In addition, there is a problem with getting the various electrodes into lateral positions: once the stylet is removed, the preset sigmoidal shape returns, but only until the lateral forces generated by the preset curves equal the strength of various unpredictable adhesions between the dura and the vertebral bones or ligamentum flavum to resist the forces. In practice, since such leads are near the delicate spinal cord and flexible dura, they must have a high degree of flexibility once the straightening stylet is removed, and this may prevent achievement of the degree of lateral electrode positioning that is desired.
Conducting coils have been used in at least parts of leads to assist defibrillation of the heart (Smits & Camps, U.S. Pat. No. 5,105,826; Holleman, Sandstrom, Rugland & Williams, U.S. Pat. No. 4,971,070). While these have a degree of flexibility and even sigmoidal or spiraling shapes, they were designed to not change their shape, nor will they pass through a Tuohy needle lumen of 2.0 mm or less. Another conducting coil was built to have two or more alternating, generally coplanar curves to act as a defibrillation device inside the heart (Stein, U.S. Pat. No. 5,405,374). However, this has a very large curving electrode, spanning an area of approximately 40 mm×40 mm, designed to touch the heart tissue at two or three places, and does not curl back upon itself in a spiral manner. Cardiac leads often have preset curves to enable the electrodes to contact specific tissue inside the heart (Kruse, Lokhoff and van Venrooij, U.S. Pat. No. 5,628,778; Hughes, U.S. Pat. No. 4,394,866). One design had a “resiliently coiled configuration”, with two 360-degree turns (Ayers, U.S. Pat. No. 5,476,498), but the curving parts are insulated, several centimeters in diameter, and used for fixation of the lead inside the heart chambers.
A shape-memory neurological lead for use in the epidural space was described in WPI Acc No: 93-342955/199343. The lead as finger-shaped wings made from shape-programmable, thermal sensitive metal and/or polymer, e.g., a bimetal or nitinol alloy. At room temperature, the wings will lay along side the lead body, which can be inserted through a needle to be positioned in the epidural space. Once implanted, at body temperature the wings will move outward into their pre-programmed shape, expanding each on in one direction, to fixate the lead optimally with respect to the boundaries of the epidural space. However, there are no conducting electrodes on the tips of the stabilization wings, and the motion is more like a person raising their arms out from the body, and not, like a person with outstretched arms curling up their fingers to form fists.
Siekmeyer and van Erp (U.S. Pat. No. 5,846,196) describe a temporary multielectrode cardiac mapping probe. The probe is believed to likely have a larger diameter than will fit through the lumen of an epidural Tuohy needle (about 2.0 mm maximum). In one embodiment, two member wires are advanced out of a confining sheath inside a heart chamber, and due to their preset elastic curves, expand to stretch out a sheet array of many recording electrodes that was folded or rolled into a compact shape inside the sheath. The electrodes are each of a fixed 2-dimensional size and rectangular shape. Since the device was not intended to be permanently implanted in the human body, the advanced members are withdrawn back into the sheath after the mapping or ablation procedure is done, collapsing by rolling or folding the sheet of electrodes again to fit in the narrow width sheath. The device has the ability to carry electrodes to more lateral positions than the width of the sheath. However, the sheath must be wide enough to accommodate the widths of numerous hard, metal electrodes when the sheet of them is made compact again. If those electrodes were of the size required for tissues stimulation, and not recording electrodes, the sheath would be 10 mm in diameter or larger. The planar sheet of electrodes may have a backing of shape memory material, perhaps made of nitinol, which also can change its conformational shape due to change in temperature inside versus outside the body, or by means of heating elements.
Chilson and Smith (U.S. Pat. No. 4,699,147) also described a cardiac mapping device that had four wires each with multiple recording electrodes, that will move apart in their middle region once they are deployed out of a sheath, forming a 3-dimensional surface, but it is similar to the device in the '196 patent, and will not perform any better for chronic tissue stimulation.
However, an optimal permanently implantable lead for tissue stimulation must have several additional features for use in the human body. It must allow the placement and use of substantially large conducting electrodes that are needed to safely and reliably pass stimulation electrical pulses of adequate amplitudes to excite tissue cells over indefinitely long periods of time, typically each about 2.0×4.0 mm or larger. To greatly minimize surgical trauma during implantation, the lead should be able to have the electrodes assume a 1-dimensional shape that is very narrow (less than 0.5 mm) inside the lead body (or sheath) for passage through a small catheter or Tuohy needle, and to assume a 2-dimensional shape when outside the lead body. Since there may be considerable deposits of fibrosis or scar tissue around each electrode within a few months of permanent implantation, if necessary, the lead should be able to be removed by gentle traction on the lead body, and have all parts easily disengage from the tissue, again without major surgical trauma.
King, Rise, Schendel and Schallhorn (U.S. Pat. No. 6,161,047) described seven lead designs that are compact and can be inserted through a sheath or Tuohy needle, and can be expanded in situ or even collapsed and removed through the lead body or sheath. Some of these use preset elastic materials to help the lead expand once it is in a position where expansion is safe, i.e., in a tissue space in the body. However, in each instance the conducting electrodes are metallic with a permanent, sizeable 2-dimensional surface at all times.
Furthermore, many of the current designs of implanted epidural stimulation leads do not have sufficient flexibility to function well in areas of great mechanical movement. For example, epidural stimulation leads in the cervical spinal cord are under great movement due to flexing of the neck. Percutaneous leads, and even paddle leads, can deliver paresthesia (the tingling feeling of stimulation that is necessary for pain relief). However, with currently available models, after implant the paresthesia may vary from nonexistent to very painful (too intense) during modest movements of the head. This is most frustrating to the patient, and prevents use of stimulation during sleep, when it may be most needed. Practitioners have gone to great lengths, and extensive surgery, to suture small paddles to the dura mater for cervical applications. An implantable lead with an array of electrodes that is very flexible and that even can urge each electrode toward the dura mater independently would be a very useful for epidural stimulation in the cervical region.
Finally, the dura mater is curved. Paddle leads generally are flat, so it is possible that several of their electrodes might not be touching the dura mater at all times. If some of them should be several millimeters away from the dura mater, scar tissue or even the fat cells that are found in the epidural space might become lodged between the dura mater and the electrodes, greatly diminishing the efficiency of the stimulation due to higher impedance for current that might otherwise pass into the spinal cord.